Added to my Automatons page, a perpetual table of equinoxes and solstices. It always shows the current year's dates and times, plus ten years before, and ten years after. The computations are performed using the algorithm described by Jean Meeus in the second edition of his book Astronomical Algorithms, pp. 177-179. The most significant source of error is in the conversion from dynamical time to universal time, because the correct conversion value must be determined by observation, therefore, when values are required beyond the available observations, they must be extrapolated from the previously observed values.
(Originally, I embedded the web page that produces the equinoxes and solstices table within this page, as I've previously done with the clock and other SVG experiments, but the quality of browser support for embedding one page in another varied so widely, that it wasn't worth doing.)